1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. More specifically, the present invention relates to a semiconductor device in which a semiconductor layer is separated for different semiconductor elements, and a manufacturing method thereof.
2. Description of the Related Art
As a typical example of a semiconductor element in which a semiconductor layer is separated, a top view and cross-sectional views of a common thin film transistor are shown in FIGS. 24A to 24D. FIG. 24A is a top view of the thin film transistor, FIG. 24B is a cross-sectional view taken along a line A1-B1 in FIG. 24A, FIG. 24C is a cross-sectional view taken along a line A2-B2 in FIG. 24A, and FIG. 24D is an enlarged view of an end portion 25 of a semiconductor layer 32 in FIG. 24C. As shown in FIGS. 24B to 24D, in the thin film transistor, an insulating layer 31 functioning as a base film is formed over a substrate 30; the semiconductor layer 32, which includes a channel formation region 32a and impurity regions 32b and 32c each functioning as a source region or drain region, is formed over the insulating layer 31; an insulating layer 33 functioning as a gate insulating film is formed over the semiconductor layer 32 and the insulating layer 31; and a conductive layer 34 functioning as a gate electrode is formed over the insulating layer 33.
In a manufacturing process of the thin film transistor shown in FIGS. 24A to 24D, in the case where the insulating layer 33 functioning as a gate insulating film is formed over the semiconductor layer 32 which is selectively etched, coverage of the insulating layer 33 decreases in the end portion 25 of the semiconductor layer 32. In a portion where the film thickness of the insulating layer 33 is thin, the electric field intensity of a gate voltage increases and stress for the gate voltage increases, which adversely affects the withstand voltage and reliability of the thin film transistor.
In addition, stress of the substrate and each thin film concentrates on the end portion 25 of the semiconductor layer 32, which causes a problem in that the element characteristics fluctuate.
As a method for improving the decrease in coverage of the gate insulating film caused by unevenness of the end portion of the semiconductor layer 32, a method in which an end portion of an active layer is tapered is used (Patent Document 1: Japanese Published Patent Application No. 2005-167207)
On the other hand, in a thin film transistor that constitutes a circuit which needs to operate at a high speed, it is preferable that the channel length be short and the film thickness of the gate insulating film be thin. Therefore, the film thickness of the gate insulating film is as thin as several tens of nanometers.